In general, files having digital data, such as documents, images, pictures, sounds, can be easily exchanged between a plurality of computers connected to a network, such as the Internet, intranets, extranets, LAN (Local Area Network).
Such files may include important files that are highly confidential and the distribution of which over the network should be restricted, such as personal information (hereinafter, refereed to as “confidential files”). Proliferation (leakage, leak) of such confidential files over the network should be prevented.
For this purpose, various techniques for preventing proliferation of confidential files over networks have been adapted.
For example, Cited Reference 1 listed below discloses a technique in which a file handles data elements as objects, the technique allows or prohibits usage of each object, and distributes the file by deleting objects under certain circumstances.
Furthermore, Cited Reference 2 listed below discloses a technique in which, when an electronic file to be written or sent in or to an external medium (another terminal) is determined as a personal information file, writing/transmission of that personal information file is prohibited, or the personal information file is written or sent in or to the external medium (another terminal) after the personal information file is converted into an encrypted file.    Patent Document 1: Japanese Laid-Open Patent Application No. 2002-007217    Patent Document 2: Japanese Laid-Open Patent Application No. 2007-034651